Ocean bottom seismometer package

ABSTRACT

A marine seismic exploration method and system comprised of continuous recording, self-contained ocean bottom pods characterized by low profile casings. An external bumper is provided to promote ocean bottom coupling and prevent fishing net entrapment. Pods are tethered together with flexible, non-rigid, non-conducting cable used to control pod deployment. Pods are deployed and retrieved from a boat deck configured to have a storage system and a handling system to attach pods to cable on-the-fly. The storage system is a juke box configuration of slots wherein individual pods are randomly stored in the slots to permit data extraction, charging, testing and synchronizing without opening the pods. A pod may include an inertial navigation system to determine ocean floor location and a rubidium clock for timing. The system includes mathematical gimballing. The cable may include shear couplings designed to automatically shear apart if a certain level of cable tension is reached.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims benefit to U.S.patent application Ser. No. 14/526,333, filed Oct. 28, 2014, titled“OCEAN BOTTOM SEISMOMETER PACKAGE”, which is a continuation of andclaims benefit to U.S. patent application Ser. No. 13/565,445, filedAug. 2, 2012, titled “OCEAN BOTTOM SEISMOMETER PACKAGE”, now issued asU.S. Pat. No. 8,879,362, which is a continuation of and claims benefitto U.S. patent application Ser. No. 13/533,011, filed Jun. 26, 2012,titled “OCEAN BOTTOM SEISMOMETER PACKAGE,” which is a continuation ofU.S. patent application Ser. No. 13/166,586, filed Jun. 22, 2011, nowissued as U.S. Pat. No. 8,228,761, titled “OCEAN BOTTOM SEISMOMETERPACKAGE,” which is a continuation of U.S. patent application Ser. No.12/838,859, filed Jul. 19, 2010, now issued as U.S. Pat. No. 7,990,803,titled “DEPLOYMENT AND RETRIEVAL METHOD FOR SHALLOW WATER OCEAN BOTTOMSEISMOMETERS,” which is a divisional of U.S. patent application Ser. No.12/004,817, filed Dec. 21, 2007, now issued as U.S. Pat. No. 7,804,737,titled “MARINE VESSEL WORKING DECK FOR HANDLING OF SHALLOW WATER OCEANBOTTOM SEISMOMETERS,” which is a continuation of U.S. patent applicationSer. No. 11/592,584, filed Nov. 3, 2006, now issued as U.S. Pat. No.7,724,607, titled “METHOD AND APPARATUS FOR SEISMIC DATA ACQUISITION,”which is a divisional of U.S. patent application Ser. No. 10/448,547,filed May 30, 2003, now issued as U.S. Pat. No. 7,310,287, titled“METHOD AND APPARATUS FOR SEISMIC DATA ACQUISITION,” each of which arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the field of seismic exploration. Moreparticularly, the invention relates to a method and apparatus forseismic exploration, and most particularly to marine seismic explorationutilizing ocean bottom seismometer systems.

Seismic exploration generally utilizes a seismic energy source togenerate an acoustic signal that propagates into the earth and ispartially reflected by subsurface seismic reflectors (i.e., interfacesbetween subsurface lithologic or fluid layers characterized by differentelastic properties). The reflected signals (known as “seismicreflections”) are detected and recorded by seismic receivers located ator near the surface of the earth, thereby generating a seismic survey ofthe subsurface. The recorded signals, or seismic energy data, can thenbe processed to yield information relating to the lithologic subsurfaceformations, identifying such features, as, for example, lithologicsubsurface formation boundaries.

Typically, the seismic receivers are laid out in an array, wherein thearray of seismic receivers consist of a single string of receiversdistributed along a line in order to record data from the seismiccross-section below the line of receivers. For data over a larger areaand for three-dimensional representations of a formation, multiplestrings of receivers may be set out side-by-side, such that a grid ofreceivers is formed. Often, the receivers within an array are remotelylocated or spread apart. In land seismic surveys for example, hundredsto thousands of receivers, called geophones, may be deployed in aspatially diverse manner, such as atypical grid configuration where eachstring extends for 1600 meters with detectors spaced every 50 meters andthe successive strings are spaced 500 meters apart. In marine surveys, atowed streamer having receivers, called hydrophones, attached theretomay trail up to 12,000 meters behind the tow vessel.

Generally, several receivers are connected in a parallel-seriescombination on a single twisted pair of wires to form a single receivergroup or channel. During the data collection process, the output fromeach channel is digitized and recorded for subsequent analysis. In turn,the groups of receivers are usually connected to cables used tocommunicate with the receivers and transport the collected data torecorders located at a central location. More specifically, when suchsurveys are conducted on land, cable telemetry for data transmission isused for detector units required to be interconnected by cables. Othersystems use wireless methods for data transmission so that theindividual detector units are not connected to each other. Still othersystems temporarily store the data until the data is extracted.

While the fundamental process for detection and recording of seismicreflections is the same on land and in marine environments, marineenvironments present unique problems due to the body of water overlayingthe earth's surface, most notably the high pressure of deep wateractivities and the corrosive environment of salt water activities. Inaddition, even simple deployment and retrieval is complicated sinceoperations must be conducted off the deck of a seismic explorationvessel, where external elements such as wave action, weather and limitedspace can greatly effect the operation.

In one common method of marine seismic exploration, seismic operationsare conducted at the surface of the water body. Marine vessels towstreamers in which are embedded hydrophones for detecting energyreflected back up through the water column. The streamers are typicallycomprised of hydrophone strings, other electrical conductors, andmaterial for providing near neutral buoyancy. The streamers are made tofloat near the water's surface. The same or other similar marine vesselstow acoustic energy sources, such as air guns, to discharge energypulses which travel downwardly into subsurface geologic formationsunderlying the water.

Systems placed on the ocean bottom floor have also been in use for manyyears. These devices are typically referred to as “OBC” (Ocean BottomCabling) or “OBS” (Ocean Bottom Seismometer) systems. The prior art hascentered on three main groups of ocean bottom apparatus to measureseismic signals at the seafloor. The first type of apparatus is an OBCsystem, similar to the towed streamer, which consists of a wire cablethat contains geophones and/or hydrophones and which is laid on theocean floor, where the detector units are interconnected with cabletelemetry. Typically, a seismic vessel will deploy the cable off the bowor stern of the vessel and retrieve the cable at the opposite end of thevessel. OBC systems such as this can have drawbacks that arise from thephysical configuration of the cable. For example, when three-dimensionalgeophones are employed, because the cable and geophones are not rigidlycoupled to the sediment on the ocean floor, horizontal motion other thanthat due to the sediment, such as for example, ocean bottom currents,can cause erroneous signals. In this same vein, because of its elongatedstructure, OBC systems tend to have satisfactory coupling only along themajor axis of the cable when attempting to record shear wave data. Inaddition, three ships are required to conduct such operations since, inaddition to a seismic energy source vessel, a specially equipped vesselis necessary for cable deployment and a separate vessel is needed forrecording. The recording vessel is usually stationary attached to thecable while the deployment vessel is generally in constant motion alongthe receiver line deploying and retrieving cable. Because the recordingvessel is in constant physical contact with the cable, the effortrequired to maintain the vessel's position, wave action and oceancurrents can generate great tension within the cable, increasing thelikelihood of a broken cable or failed equipment, as well as theintroduction of signal interference into the cable. Finally, such cablesystems have a high capital investment and are generally costly tooperate.

A second type of recording system is an OBS system in which a sensorpackage and electronics package is anchored to the sea floor. The devicedigitizes the signals and typically uses a wire cable to transmit datato a radio unit attached to the anchored cable and floating on the watersurface. The floating transmitter unit then transmits the data to asurface vessel where the seismic data are recorded. Multiple units aretypically deployed in a seismic survey.

A third type of seismic recording device is an OBS system known asSeafloor Seismic Recorders (SSR's). These devices contain the sensorsand electronics in sealed packages, and record signals on the seafloor.Data are retrieved by retrieving the device from the seafloor. Suchdevices are typically re-usable. The focus of the present invention ison SSR type of OBS systems.

SSR type OBS systems generally include one or more geophone and/orhydrophone sensors, a power source, a seismic data recorder, a crystaloscillator clock, a control circuit, and, in instances when gimbaledgeophones are used and shear data are recorded, a compass or gimbal.Except to the extent power is provided from an outside source via acable, the power source is generally a battery package. To the extentprior art OBS systems have utilized on-board batteries, as opposed toexternal cabling, to supply power, the prior art batteries have beenlead-acid, alkaline or non-rechargeable batteries. All of the OBSsystems of the prior art generally require that the individual units beopened up for various maintenance, quality control and data extractionactivities. For example, data extraction from prior art units requirethe units be physically opened or disassembled to extract data.Likewise, the unit must be opened up to replace spent batteries.

With respect to the timing function of the OBS system, synchronizationbetween the timing of the sensor data and the firing of the seismicenergy source or shot is critical in order to match a seismic sourceevent with a reflection event. In the past, various crystal oscillatorclocks have been used in OBS systems for this function. The clocks arerelatively inexpensive and accurate. One drawback to such prior artclocks, however, is that the clock crystals are subject to gravitationaland temperature effects. These gravitational and temperature effects cancause a frequency shift in the oscillator frequency, thereby resultingin errors in the seismic data. In addition, since the crystals aresubject to gravitational effects, orientation of the OBS system caneffect operation of the clock. Since the clock is typically securedwithin the OBS package so as to be correctly oriented when the OBSsystem is properly oriented on the ocean floor, any misorientation ofthe OBS system on the ocean floor can result in clock inaccuracies.Finally, such clocks often are characterized by drift and time shiftsdue to temperature changes and aging, which again, can causeinaccuracies in the recorded seismic data. While it may be possible thatmathematical corrections could be made to the data to account fortemperature aging and time shifts, there is no prior art device thatcorrects for gravitational effects on the crystal clock. At most, theprior art only corrects for effects of temperature on the crystalclocks.

More modem OBS systems may also include a mechanical device to correctfor tilt, namely a gimbal. A gimbal is a device that permits freeangular movement in one or more directions and is used to determineorientation of the OBS system on the ocean floor. Orientation datagenerated by the gimbal can then be used to adjust the seismic datarecorded by the geophones. To the extent the prior art utilizes gimbals,they are most often incorporated as part of the geophone itself, whichare referred to as “gimbaled geophones,” One drawback to thesemechanical gimbals of the prior art is the limited angular orientationpermitted by the devices. For example, at least one of the prior artdevices permit a gimbal roll of 360° but is limited in gimbal pitch to30°. For this device, in order for such prior art gimbals to functionproperly, the OBS system itself must settle on the ocean floor insubstantially the desired position. To the extent the OBS system is notoriented at least substantially in the horizontal, such as settling onits side or upside down, the mechanical gimbal of the prior art may notfunction properly. Other gimbaled devices of a mechanical nature are notlimited by 30°, however, in such mechanically gimbaled devices,mechanical dampening in the device can deteriorate the fidelity of therecorded signal. Finally, gimballing of a geophone is expensive andrequires more space than a non-gimballed geophone. For OBS systems thatutilize multiple geophones, it may be impractical to gimbal thegeophones due to size and space requirements.

As with orientation, the location of OBS system on the ocean floor isnecessary to properly interpret seismic data recorded by the system. Theaccuracy of the processed data depends in part on the accuracy of thelocation information used to process the data. Since conventionallocation devices such as GPS will not operate in the water environments,traditional prior art methods for establishing the location of the OBSsystems on the ocean floor include sonar. For example, with a sonarsystem, the OBS device may be “pinged” to determine its location. In anyevent, the accuracy of the processed data is directly dependent on theprecision with which the location of the OBS system is determined. Thus,it is highly desirable to utilize methods and devices that will producedependable location information. In this same vein, it is highlydesirable to ensure that the planned positioning of the OBS device onthe ocean floor is achieved.

With respect to operation of the aforementioned OBS systems, the priorart systems generally require some externally generated control commandin order to initiate and acquire data for each shot. Thus the seismicreceiver units must be either physically connected to the centralcontrol recording station or “connectable” by wireless techniques. Asmentioned above, those skilled in the art will understand that certainenvironments can present extreme challenges for conventional methods ofconnecting and controlling the detectors, such as congested or deepmarine areas, rugged mountain areas and jungles. Difficulties may alsoarise in instances where the receiver array is periodically moved tocover a larger area.

Whatever the case, each type of connection, whether via a physical cableor through wireless techniques, has its own drawbacks. In cabletelemetry systems, large arrays or long streamers result in largequantities of electrically conductive cabling that are expensive anddifficult to handle, deploy or otherwise manipulate. In instances whereocean bottom cabling is used, the corrosive environment and highpressures often require costly cable armoring in water depths over 500feet. Furthermore, conventional cabling also requires a physicalconnection between the cable and the sensor unit. Since it is generallynot practical to hard wire sensors on a cable, the more conventionaltechnique is to attach cabling to sensors using external connectionsbetween the cable and the sensor. This point of the connection betweenthe cable and the sensor is particularly vulnerable to damage,especially in corrosive, high pressure marine environments. Of course,with systems that are physically cabled together, it is much easier toprovide power to the sensors, to synchronize sensors with the shot timeand with each other and to otherwise control the sensors.

It should be noted that whether for cabled or wireless systems, whereexternal cabling is required to connect the sensor package of theequipment with the recording and/or radio telemetry packages of theunit, many of the aforementioned drawbacks exist. Specifically, the OBSsystems of the prior art are comprised of separate sensing andrecording/radio telemetry units or packages mounted on a carriage. Theseparate units have external connectors that are cabled together,presenting many of the same problems as cabling from the central controlon the surface of the water. The primary reason for the separationbetween the sensing units, i.e., the geophone package, and the remainderof the electronics is the need to ensure that the geophones areeffectively coupled to the ocean floor.

In cases where either wireless technology is utilized or operation ofsensors is through pre-programming, control of the sensors becomes moredifficult. For example, ensuring that recording is synchronized with theshot timing is crucial since the individual sensors are not wiredtogether as described above. Hence the need for accurate on-board clocksas mentioned above. In this regard, activating each unit for sensing andrecording at the appropriate time must coincide with the shot. Ensuringthat the units are sufficiently powered has also heretofore been aconcern. Many prior art patents have focused on techniques and mechanisms for powering up sensors during data acquisition and recording andpowering down the sensors during dormant periods.

Various attempts have been made to address some of the above-mentioneddrawbacks. For example, a seafloor seismic recorder is described in U.S.Pat. No. 5,189,642. This patent discloses an elongated, upright chassisformed of spaced apart, horizontal ring plates connected by vertical legmembers. Each leg member is formed of nested tubes that can sliderelative to one another and that are secured to one another by a clampmechanism. Releasably attached to the lower plate is a ballast ring.Also attached to the lower plate is the geophone package. Attached tothe upper plate is a foam buoy. A control package extends down from theupper plate. The control package houses a power source, a seismic datarecorder, a compass and a control circuit. An external hard wireelectrically connects the control package with the geophone package. Thesystem does not utilize any hard-wired communications link to thesurface monitoring station but utilizes acoustical or preprogrammedmeans for controlling the unit. When released into the water, theballast ring is supposed to provide sufficient mass to maintain thesystem upright and couple the geophones to the ocean floor uponsettling. To minimize the likelihood of geophone noise produced by waveor water current motion acting against the buoy and control package,once the system is coupled to the ocean bottom, the clamp mechanism oneach leg is released, allowing the control package and buoy to slideupward on the nested legs, isolating the geophones from the other partsof the system. Once seismic recording is complete, the ballast ring isthen released from the chassis, and the system rises to the watersurface under the positive buoyancy of the ballast. Acoustictransducers, a radio beacon and strobe light are provided to permit thesystem to be located and retrieved.

Another marine seismic data recording system is taught in U.S. Pat. No.6,024,344. This patent teaches a method for deploying and positioningseismic data recorders in deep water. From a surface vessel, datarecorders are attached to a semi-rigid wire which is deployed into thewater. Due to the rigid nature of the wire, it functions to define afixed interval between recorders as the recorders and wire sink to theseafloor. The wire also provides electrical communication fat power orsignals between adjacent recorders and between recorders and the vessel.Once the recorders are in place, they are activated either by way of apreset clock or by utilizing a control signal transmitted through thewater or through the wire. Upon completion of data gathering, the wireand recorders are retrieved. Deployment is accomplished utilizing acable engine positioned on the surface vessel. As shown in FIG. 1 of the'344 patent, deployment occurs over the stern of the vessel as it movesin a direction away from the wire and recorders. This patent alsoteaches the need to store the recorders in a sequential manner tofacilitate deployment and to track the seafloor location of the OBSsystem during data collection.

GeoPro offers a self-contained, i.e., cable-less, OBS system comprisedof a 430 mm diameter glass sphere in which is enclosed all electricalcomponents for the system, including batteries, a radio beacon, aseismic data recording unit, an acoustic release system, a deep seahydrophone and three gimbal mounted geophones. The sphere is mounted ona weighted skid that counteracts the buoyancy of the sphere and anchorsthe OBS system to the sea bed. The geophones are positioned in thebottom of the sphere adjacent the skid. To recover the OBS system uponcompletion of data collection, an acoustical command signal istransmitted to the sphere and detected by the deep sea hydrophone. Thesignal activates the acoustic release system which causes the sphere toseparate from the weighted skid, which remains on the sea floor. Underpositive buoyancy of the sphere, the free-floating system rises to theocean surface, where the radio beacon transmits a signal for locatingand retrieving the sphere. One drawback to this particular design isthat the geophones are not coupled directly to the ocean floor. Rather,any seismic signal recorded by the geophones must pass through the skidand the bottom of the sphere, and in so doing, are subject to noise andother, distortions described above. It should be noted that thispackaging design is representative of many of the cylinder and sphereshapes utilized in the prior art since it is well known that such shapesare more effective in withstanding the high pressures likely to be foundin ocean environments.

K.U.M. and SEND offer a cable-less OBS system comprising a frame havinga rod at the top and forming a tripod at the bottom. A foam flotationdevice is attached to the rod. An anchor is fixed to the lower portionof the tripod and secures the frame to the sea floor. Pressure cylindersmounted on the tripod portion of the frame contain seismic recorders,batteries and a release system. A hydrophone is attached to the frame inorder to receive command signals from the ocean surface and activate therelease system. Also attached to the frame is a pivotally mounted cranearm to which is releasably attached a geophone unit. During deployment,the crane arm is initially maintained in a vertical position with thegeophone unit attached to the free end of the arm. When the framecontacts the sea floor, the crane arm pivots out from the frame andreleases the geophone unit onto the sea floor approximately 1 meter fromthe frame system. A hard wire permits electrical communication betweenthe geophone unit and the recorders. The geophone unit itself is anapproximately 250 mm diameter, non-symmetrical disk which is flat on oneside and domed on the opposite side. The flat side of the geophone unitis grooved and contacts the sea floor when released by the crane arm.Upon completion of data gathering, an acoustic signal activates therelease system, which causes the anchor to be detached from the framesystem. The foam flotation device causes the frame system and geophoneto rise to the ocean surface where the system can be located using theradio beacon and retrieved.

SeaBed Geophysical markets a cable-less OBS system under the name CASE.This system is comprised of a control unit, i.e., electronics package,and a node unit or geophone package connected to each other by a cable.Both the control unit and the node unit are carried on an elongatedframe. The control unit is comprised of a tubular body which containsbatteries, a clock, a recording unit and a transponder/modem forhydro-acoustic communication with the surface. The node unit iscomprised of geophones, a hydrophone, a tilt meter and a replaceableskirt, wherein the skirt forms a downwardly open cylinder under thegeophone unit. The node unit is detachable from the elongated frame andcontrol unit, but remains in communication with the control unit viaexternal cabling. The use of a tubular body such as this is veryrepresentative of prior art designs because the system packaging must bedesigned to withstand the high pressures to which the device is exposed.During deployment, the entire unit is dropped to the sea floor, where aremotely operated vehicle (separate from the OBS system) is used todetach the node unit from the frame and plant the node unit into theseafloor, pushing the open-ended skirt into the seafloor sediment. Theelongated frame includes a ring to which a deployment and retrievalcable can be attached. The communication transducer and modem areutilized control the system and transmit seismic data to the surface.

Each of the referenced prior art devices embodies one or more of thedrawbacks of the prior art. For example, the OBS system of U.S. Pat. No.5,189,642, as well as the devices of GeoPro and K.U.M./SEND are uprightsystems that each have a relatively tall, vertical profile. As such,seismic data collected by these systems is subject to noise arising fromwater movement acting against the devices. In addition, it has beenobserved that shear motion caused by movement of the ocean floor undersuch a tall profile OBS system can cause rocking motion of the OBSsystem, particularly as the motion translates from the bottom to the topof the unit, further deteriorating fidelity of the recorded data.Furthermore, these prior art devices are all asymmetrical, such thatthey can be positioned in only a single orientation. Typically this isachieved by heavily weighting one end of the OBS carriage. However, sucha device likely must pass through hundreds of feet of water and contactan often rugged, uneven ocean floor that may be scattered with debris.All of these factors can result in mis-orientation of the system as itsettles on the ocean floor, thereby effecting operation of the system.For example, to the extent such a prior art OBS system settles on itsside, the geophones will not couple with the ocean floor at all,rendering the device unusable. In addition, incorrect orientation couldinterfere with the system's release mechanism, jeopardizing recovery ofthe system.

The tall profile of these prior art systems is also undesirable becausesuch units lend themselves to becoming entangled in fishing lines,shrimping nets, various types of cables or other debris that might bepresent in the vicinity of the seismic recording activity.

On the other hand, prior art systems that have a smaller profile, suchas ocean bottom cables, tend to have poor coupling ability or requireexternal assistance in placement utilizing expensive equipment such asROVs. For example, the elongated shape of ocean bottom cables results in“good” coupling in only a single orientation, namely along the majoraxis of the cable. Furthermore, even along the major axis, because ofthe small surface area of actual contact between the cable and the oceanfloor, coupling can be compromised due to a rugged ocean bottom or otherobstacles on or near the ocean floor.

Another drawback to these prior art systems is the need to activate anddeactivate the units for recording and operation. This generallyrequires a control signal from the surface vessel, typically eithertransmitted acoustically or through a cable extending from the surfaceto the unit. External control of any type is undesirable since itrequires signal transmission and additional components in the system.While acoustical transmission can be used for some data transmission, itis generally not reliable to use for synchronization purposes due tounknown travel path variations. Of course, any type of control signalcabling for transmission of electrical signals is undesirable because itadds a level of complexity to the handling and control of the unit andrequires external connectors or couplings. Such cabling and connectorsare particularly susceptible to leakage and failure in the highpressure, corrosive environment of deep ocean seismic exploration.

A similar problem exists with units that utilize external electricalwiring to interconnect distributed elements of the unit, such as istaught in U.S. Pat. No. 5,189,642 and similar devices where the geophonepackage is separate from the electronics package. Furthermore, to theextent the electronics of a system are distributed, the likelihood ofmalfunction of the system increases.

Many of the prior art systems also use radio telemetry rather thanrecording data on-board the unit, to collect the data. Such systems, ofcourse, have limitations imposed by the characteristics of radiotransmission, such as radio spectrum license restrictions, rangelimitations, line-of-sight obstructions, antenna limitations, data ratelimitations, power restrictions, etc.

Those OBS units that utilize flotation devices for retrieval areundesirable because the typical decoupler device adds additional expenseand complexity to the units, and generally must be activated in order torelease the systems to the surface. In addition, such systems typicallydiscard part of the unit, namely the weighted anchor or skid, leaving itas debris on the ocean floor. During deployment, since they arefree-floating, such systems are difficult to position in a desiredlocation on the ocean floor. Notwithstanding the above-mentionedpossibility of malfunction due to misorientation, during retrieval, thefree-floating systems are often difficult to locate and have been knownto be lost-at-sea, despite the presence of radio signals and beacons.Likewise, in rough seas, the units prove unwieldy to snare and lift onboard, often colliding with the boom or vessel hull and potentiallydamaging the system.

In this same vein, handling of the units, both during deployment andretrieval, has proven difficult. To the extent a rigid or semi-rigidcable system is utilized to fix distances and position individualrecorder units, such cables are inflexible, extremely heavy anddifficult to manipulate. Such cables do not lend themselves tocorrections during deployment. For example, as explained above, adesired grid layout identifies specific positions for individual unitsalong a line. If a deployment vessel drifts or otherwise causes a cablebeing laid to be positioned off of the desired line, the vessel at thesurface must reposition to cause the cable to get back on line. However,because of the rigid nature of the cable, the mispositioned portion ofthe cable will result in all of the remaining units on the cable to bemispositioned along the desired line.

Furthermore, current procedures utilized in the prior art to retrievecables tends to place undue stress on the cables. Specifically, thewidely accepted method for retrieval of a cable line from the oceanfloor is to either back down over a line or drive the boat down the lineretrieving the cable over the bow of the vessel. This is undesirablebecause the speed of the vessel and the speed of the cable winch must becarefully regulated so as not to overtension or pull the cable. Suchregulation is often difficult because of the various external factorsacting on the vessel, such as wind, wave action and water current.Failure to control tensioning or pulling of the cable will have theeffect of dragging the entire length of the line, as well as the unitsattached thereto, subjecting the entire line and all of the units todamage. An additional drawback to this method is that if the vessel ismoving too fast, it will cause slack in the cable and the cable willfloat under the vessel, where it can become entangled in the vessel'spropellers.

Finally, nowhere in the prior art is there described a back-deck systemfor handling the above-described OBS units, whether it be storage of theunits or deploying and retrieving the units. As the size of deep waterseismic recorder arrays become larger, the need for a system forefficiently storing, tracking, servicing and handling the thousands ofrecorder units comprising such an array becomes more significant.Additional surface vessels are costly, as are the personnel necessary toman such vessels. The presence of additional personnel and vessels alsoincreases the likelihood of accident or injury, especially in open-seaenvironments where weather can quickly deteriorate.

Thus, it would be desirable to provide a seismic data collection systemthat does not require external communication/power cabling, either fromthe surface or on the seismic data collection unit itself, nor any typeof external control signal for operation. In other words, the unitshould operate on a “drop and forget” basis. Likewise, the device shouldbe easily serviced without the need to open the device to performactivities such as data extraction, quality control and powerreplenishment. The device should also be designed to withstand thecorrosive, high pressure environment common in deep water marineapplications. The unit should be configured to minimize the effects ofnoise arising from ocean currents, and maximize coupling between thedevice and the ocean floor. In this same vein, the device should bedesigned to properly orient itself for maximum coupling as the devicecontacts the ocean floor, without the assistance of external equipmentsuch as ROVs, and minimize the likelihood of misorientation. Likewise,the device should be less susceptible to snaring or entrapment byshrimping nets, fishing lines and the like.

The device should include a timing mechanism that is not susceptible toorientation. Similarly, orientation should not effect gimballing of thegeophones.

The device should be easily deployable, yet able to be placed at acertain location with a high degree of confidence. Likewise, the deviceshould be easily retrievable without the need for flotation devices orrelease mechanisms, nor should parts of the unit be left in the oceanduring retrieval. Further, there should be a device and retrievalprocedures that minimize potentially damaging tension in the cableconnecting the seismic units.

There should also be provided a system for readily handling the hundredsor thousands of recorder units that comprise an array for deployment inocean environments. Such a system should be able to deploy, retrieve,track, maintain and store individual recorder units while minimizingmanpower and the need for additional surface vessels. The system shouldlikewise minimize potential damage to the individual units during suchactivity. Likewise, it would be desirable to include safety devices inthe system to minimize harm to personnel handling the recorder units.

SUMMARY OF THE INVENTION

The present invention provides a system for collecting seismic data inmarine environments by deploying multiple, continuous operating,wireless, self-contained ocean bottom sensor units or pods, eachcharacterized by a symmetrical, low profile casing, and a uniqueexternal bumper to promote ocean bottom coupling and prevent entrapmentin fishing nets. The pods are attached to one another utilizing aflexible, non-rigid, non-conducting cable that is used to controldeployment of the pods through the water. The pods are deployed andretrieved from the uniquely configured deck of marine vessel, whereinthe deck is provided with a conveyor system and a handling system toattach and detach individual pods from the non-rigid cable. In oneembodiment, as part of the deck configuration, the individual pods arerandomly stored in juke box fashion in slotted racks. When seated withinthe slot of a rack, the seismic data previously recorded by the pod canbe retrieved and the pod can be charged, tested, re-synchronized, andoperation can be re-initiated without the need to open the pod. Inanother embodiment, the individual pods are stored in stacked, rotatingcarousels that permit seismic data previously recorded by the pods to beretrieved and the pods to be charged, tested, re-synchronized, andoperation can be re-initiated without the need to open the pod. Duringdeployment and retrieval, the non-rigid cable and pods attached theretoare handled so as to minimize the likelihood of tension developingwithin the deployed line by virtue of movement of the surface vessel.This includes a uniquely configured non-rigid cable system designed toautomatically shear apart if a certain level of tension is reached inthe cable.

More specifically, each individual sensor unit is comprised of adisk-shaped, water tight case formed of two parallel, circular platesjoined around their peripheries by a shallow wall, thereby forming apackage which is symmetrical about the axis of the plates and has a verylow height profile relative to the diameter of the plates, much in theshape of a wheel. The case is internally supported to protect theintegrity of the case from external pressure effects and to providerigid mechanical coupling between the unit case and the geophones. Inone embodiment of the invention, the unit is configured so that it willeffectively couple with the ocean floor and collect seismic datawhichever plate side it settles on, obviating many of the orientationproblems of the prior art. The plates may include ridges, projections orgrooves to enhance coupling with the ocean floor.

Disposed around the shallow wall of the unit in one embodiment is abumper having a cross section shape designed to urge the unit to settleonto one of the plate sides of the package, thereby resulting in a highdegree of coupling between the unit and the ocean floor. In at least oneembodiment, a bumper is provided and designed to prevent the unit frombecoming entangled or snared in shrimping nets or fishing lines.

The unit utilizes several different devices for connecting to a cable.In one embodiment, each unit includes an over-center latching mechanismto permit the units to be attached to a cable. In another embodiment, anattachment bracket is located off-center on the side of the case. Instill yet another embodiment, an attachment bracket is centrally locatedon one of the unit's circular plates forming the case.

The unit is self-contained such that all of the electronics are disposedwithin the case, including a multi-directional geophone package, aseismic data recording device, a power source and a clock.

In one embodiment of the invention, the clock is a rubidium clock. Therubidium clock is much less susceptible to temperature or gravitationaleffects or orientation of the unit on the ocean floor.

In another embodiment, the unit includes a crystal clock and a tiltmeter. Gravitational effects on the crystal clock are preferablycorrected on-board the unit in real time utilizing tilt meter data.

The power source is preferably rechargeable batteries that can operatein a sealed environment, such as lithium ion batteries.

Units incorporating a tilt meter may also utilize the tilt meter data toperform various functions other than crystal clock correction. Forexample, one aspect of the invention utilizes tilt meter data formathematical gimballing. Specifically, in the invention, gimballing ofthe geophones is accomplished mathematically using tilt meter data, andas such, is not subject to the orientation of the unit as arc mechanicalgimbals.

Of course, tilt meter data may also be used to determine the position ofa unit on the ocean floor as is the common use of such data in the priorart. However, unlike the prior art devices, one aspect of the inventionis to obtain and utilize tilt meter data in a time continuous fashion.Prior art units typically only determine a unit's position once at thebeginning of seismic recording. Yet it has been observed that theposition of a unit may change over the course of deployment as the unitis subject to external forces such as water currents, shrimp lines andthe like. Thus, in the invention, tilt meter data is measured as afunction of time. This is performed multiple times during operation sothat seismic data can be corrected as necessary.

With respect to corrections for tilt, timing or similar data that couldeffect the accuracy of the collected seismic data, all of the prior artdevices make such corrections at a processing center. None of the priorart devices make such corrections on-board the unit while it is deployedor even on board the deployment vessel. Thus, one method of theinvention is to make such corrections on-board the unit while it isdeployed.

The unit may also include a compass, a hydrophone, an acousticallocation transducer and/or one or more accelerometers. Compass data maybe used to provide frame of reference data for each individual unitrelative to the frame of reference for the overall survey. In oneembodiment of the invention, sensors such as accelerometers are used totrack the position of the unit as it descends through a water column andsettles on the ocean floor. Specifically, such sensors provide inertialnavigation data and record x, y and z position information as the unitis passing through the water column. This position information, alongwith initial position and velocity information, is used to determine theeventual location of the unit.

In another aspect of the invention, the unit is activated while on-boardthe seismic vessel and deactivated once pulled from the ocean, such thatit is continuously acquiring data from before the time of deployment toafter the time of retrieval. Likewise in one embodiment, the unit beginsrecording data prior to deployment in the water. Systems that areactivated and begin recording before deployment in the water are therebystabilized prior to the time when signal detection is desired. Thisminimizes the likelihood that an altered state in electronics operationwill disrupt signal detection and recording.

In another aspect of the invention, the seismic data recording deviceincludes wrap around memory and continuously records, even when not inuse. This obviates the need for initiation or start instructions,ensures that the unit is stabilized at the desired recording times, andserves to back-up data from prior recordings until such time as theprior data is written over. As long as the clock is synchronized, such arecording device is ready for deployment at any time. Furthermore,routine operations such as data collection, quality control tests andbattery charging can take place without interrupting recording. In thecase of a continuously recording unit such as this, the unit can be usedon land or in a marine environment.

Use of a non-rigid cable is an additional aspect of the invention. Whilerope may have been used in the very early prior art as a tow line forsurface floating seismic devices, heretofore, to the extent OBS systemshave been connected to one another, the prior art has utilized onlyrigid or semi-rigid wire cable. One of the reasons wire cable has beendesirable for the prior art OBS systems is the need to electricallyinterconnect the systems. In the current invention, however, flexible,non-rigid cable is utilized since the pods, as described above, operateindependently and do not require external communications or connections.The non-rigid cable of the invention is preferably formed of a syntheticfiber material, such as polyester, and is encased in a protectiveovermold, such as a polyurethane casing. In one embodiment, thenon-rigid cable is formed of a twelve stranded braided polyester core.The overmold is ribbed or grooved to reduce drag in the water.

The non-rigid cable of the invention is also useful in a uniquedeployment method for the pods. Specifically, the non-rigid cable hasonly a slightly negative buoyancy. When attached between two pods eachhaving a negative buoyancy much greater than the cable, as the twojointed pods sink down through a water column, the drag on the non-rigidcable is much greater than the drag on the units and thus acts as aparachute or brake, slowing the descent of the pods and maintaining thepods in an upright position. This is particularly desirable in unitsthat must be placed in a particular orientation, such as those unitshaving non-symmetrical bumper configurations, because the cable, whenattached to a centrally mounted connector on the top plate, functions tomaintain the orientation of the unit as it passes down through the watercolumn and settles on the ocean floor. Furthermore, since the cable ofthe invention is non-rigid, there is slack in the cable between adjacentpods. A vessel operator can utilize this slack to make corrections inthe drop location while deploying the pods.

Likewise, the non-rigid cable enhances a unique retrieval method of theinvention, wherein the cable is retrieved over the stern of the vesselas the vessel “drives down” the cable. In so doing, the drag on thecable created by the water causes the cable to parachute or billow outbehind the vessel, minimizing excessive tension on the cable andensuring that the cable is less likely to become entangled in thevessel's propellers.

On the deck of the seismic vessel, in one embodiment of the invention, astorage system includes a rack having multiple rows and columns of slotsis disposed for receipt of the individual units. Each slot includes acommunications portal such that when a unit is seated within the slot,the unit interfaces with a master control station via the communicationsportal. Through the portal, information recorded on the unit can bedownloaded, the unit batteries can be recharged, quality control checkson the unit can be conducted, recording can be re-initiated and the unitcan be reactivated. In another embodiment of the invention, a storagesystem includes stacked, u-shaped carousels. Each carousel includesrollers to permit the recording units to be moved along the path of thecarousel in conveyor type fashion until the units are positionedadjacent a communications portal. Whichever storage system is utilized,the storage systems may be configured to have the dimensions of astandard 8′×20′×8′ shipping container so that the storage systems andany seismic units stored therein, can be easily transported utilizingstandard container ships.

Each unit may include a unique identification means, such as a radiofrequency identification (RFID) tag or similar identification indicia topermit tracking of the individual units as they are handled on the deck.Likewise, as mentioned above, each unit may include an acousticallocation transducer or accelerometers to determine a unit's location onthe ocean floor. Since the individual units are self-contained, thelocation information, in association with the identification indiciaallows the units to be randomly inserted into the storage rack, butpermits data from multiple units to be retrieved and sequentiallyordered according to the previous location of the unit on the oceanfloor. Thus, the need to keep units in sequential order is obviated.Units that might have been adjacent one another on a receiver line neednot be stored next to one another in the racks.

In addition, the overall deployment and retrieval system for the unitsis substantially automated on the deck. The deck configuration includesa conveyor system running adjacent the racks and extending to the edgeof the deck adjacent the water. A robotic arm is positioned for movingthe units between the storage rack and the conveyor belt. In oneembodiment, a cable engine and cable spool/container are positioned topay out non-rigid cable so as to run adjacent the conveyor system andover the side of the vessel. As units are placed on the conveyor systemfor attachment to the non-rigid cable, the speed of the conveyor isadjusted to match the speed of the cable, permitting attachment of theunits on-the-fly. Furthermore, those skilled in the art will understandthat the payout speed of line is not constant since movement of thevessel through the water is not constant, even under calm seas and lowwind conditions. As such, in order to prevent too much tension fromdeveloping in the line, which can result in damage to the line anddragging of the units, and to permit accurate placement of the units onthe ocean floor, the speed of the line as it is paid out into the wateris constantly adjusted to compensate for the erratic and unpredictablemovement of the vessel on the water. Thus, the speed of the conveyorcarrying the units for attachment to the line must be continuallyadjusted.

In another embodiment of the invention, the conveyor intersects with thecable being paid out by the cable engine. At the intersection, a seismicunit is attached to the cable and the attached unit is subsequentlyreleased into the water. A cable grabber downstream from the attachmentstation is used to securely clamp the cable prior to attachment of aunit, thereby removing upstream line tension during attachment of theunit to the cable. The cable grabber may include a release systemrequiring an operator to use both hands in order to open the grabber,thereby minimizing danger to the operator when the unit is released andthe upstream cable is again placed under tension.

With respect to tension in the cable, the cable is sectioned and thecable sections are attached to one another utilizing a uniquelydesigned, break-away connector. The connector is comprised of first andsecond fittings that nest into each other. A shear pin is insertedthrough the nested fittings to secure the fitting together. Each fittingis attached to the end of a cable section such that when the fittingsare secured together, the cable sections form a longer length of cable.If the tension in the cable become greater than the shear limit of theshear pin, the shear pin with break away and the cable will separate.

Furthermore, while one embodiment of the invention utilizes a clampingmechanism that permits units to be clamped directly on a length ofcable, another embodiment of the invention utilizes a sleeve attached tothe cable. The clamping mechanism secures to the sleeve which is boundedby overmolded shoulders. Rather than attaching shoulders betweenadjacent lengths of cable as is common in the prior art, the sleeve ofthe invention can be clamped or placed around a length of cable andsecured in place without cutting the cable. In the embodiment, thesleeve is secured to the cable by inserting pins through the sleeve andcable in the x and y planes perpendicular to the axis of the cable.Shoulders are molded over the pins at the ends of each sleeve. While theovermolding on opposite ends of the sleeve can be used to define anattachment area along the sleeve, the sleeve may include flared endsthat further define such attachment area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away top view of the seismic recorder unit of thecurrent invention.

FIG. 2 is a front side view of the unit of FIG. 1.

FIG. 3 is a back side view of the unit of FIG. 1.

FIG. 4 is a top view of the unit of FIG. 1.

FIG. 5 is a back side view of the unit with a cross-section of therounded bumper.

FIG. 6 is a back side view of the unit with a cross-section of a wedgebumper.

FIG. 7 is a top view of the unit with the wedge bumper of FIG. 6.

FIG. 8 is elevated view of the unit with a hinged flipper.

FIG. 9 is a cut-away end view of the non-rigid cable.

FIG. 10 is a cut-away side view of shear pin connector.

FIG. 11 is an elevation view of the shear pin connector of FIG. 10.

FIG. 12 is a cut-away side view of the pod attachment cable sleeve.

FIG. 13 is an elevation view of the attachment sleeve of FIG. 12.

FIG. 14 is a side view of a seismic system deployment and retrievalvessel.

FIG. 15 is a back deck layout illustrating an automated, speed-matching,pod launcher system and pod storage system.

FIG. 16 is a side view of the juke box storage rack.

FIG. 17 is an end view of the deck layout of FIG. 15.

FIG. 18 is an elevation view of the deck layout of FIG. 15.

FIG. 19 is a back deck layout illustrating the semi-automatic podattachment system.

FIG. 20 illustrates an over-the-stern pod retrieval method.

FIG. 21 illustrates multiple units attached to a non-rigid line duringdeployment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the detailed description of the invention, like numerals are employedto designate like parts throughout. Various items of equipment, such asfasteners, fittings, etc., may be omitted to simplify the description.However, those skilled in the art will realize that such conventionalequipment can be employed as desired.

With reference to FIG. 1, there is shown a seismic data collectionsystem or pod 10 of the invention. Pod 10 is comprised of a water tightcase 12 having a wall 14 defining an internal, water-tight compartment16. Disposed within compartment 16 is at least one geophone 18, a clock20, a power source 22, a control mechanism 23 and a seismic datarecorder 24. In the embodiment, pod 10 is self-contained such that powersource 22 meets all of the power requirements of pod 10. Likewise,control mechanism 23 provides all control functions for pod 10eliminating the need for external control communications. Pod 10 isweighted to have a negative buoyancy so that it will sink towards theocean floor when deployed in a water column.

Those skilled in the art will appreciate that pod 10 is a self-containedseismic data collection system which requires no external communicationor control in order to record seismic signals. It will be further notedthat geophone 18 is internally mounted within pod 10 and thus requiresno external wiring or connection. It has been determined that utilizingthe case design described in more detail below, geophone 18 iseffectively coupled to the ocean floor such that seismic datatransmitted through pod 10 to geophone 18 is not corrupted byinterference.

While the basic elements have been described above, pod 10 may alsoinclude a compass 36 and a tilt meter 38. Furthermore, in the preferredembodiment, geophone 18 is a geophone package comprised of threegeophones to detect seismic waves in each of the x, y and a axes. Unlessspecifically indicated, all references to geophones utilized in theinvention include conventional geophones as well as other known devicesfor detecting seismic wave activity, including without limitation,accelerometers.

In another embodiment of the invention, it has been found advantageousto utilize four geophones positioned in a tetrahedral configuration suchthat each geophone measures data in multiple planes. In a standard threedimensions configuration, three geophones are positioned 90° apart fromeach other and each geophone measures signal in a single x, y or zplane. In a four geophone configuration, the geophones are orientedperpendicular to the plane of the tetrahedral faces so that eachgeophone measures portions of multiple planes in the x, y, z coordinatesystem. For example, one geophone may measure seismic data in thex-plane and z-plane. Geophone configurations of four or more geophonesare desirable because they provide for redundancy in the seismic unit inthe event of failure of a geophone in a particular plane. None of theprior art OBS systems have utilized four or more geophones to detectseismic data in the manner.

In one important aspect of the invention, clock 20 is a rubidium clack.Heretofore, rubidium clocks have not been used in seismic explorationdue in part to the expense when compared to traditional crystal drivenclocks. However, because the pod 10 of the invention is intended tooperate most effectively in one of several orientations, it is necessaryto utilize a clock that in not susceptible to orientation effects whichcan inhibit operation of traditional prior art crystal clocks.Furthermore, rubidium clocks are less susceptible to temperature andgravitational effects that can inhibit operation of prior art clocks inocean environments,

Power source 22 is preferably a lithium ion battery. To the extent priorart OBS systems have utilized on-board batteries, as opposed to externalcabling to supply power, the prior art batteries have been lead-acid,alkaline or non-rechargeable batteries. None of the prior art OBSsystems have utilized lithium ion batteries. However, because of thesealed, self-contained nature of the pod of the invention, it isdesirable to utilize a battery, such as the lithium ion type, that doesnot vent fumes and are easily rechargeable.

In FIGS. 2 and 3, one of the unique features of pod 10 can beappreciated, namely the low profile configuration of pod 10.Specifically, case 12 comprises a first plate 26 and a second plate 28jointed together along their peripheries by wall 14. In one embodimentplates 26 and 28 are disk shaped, such that the overall shape of case 12is that of a wheel. In any event, as can be appreciated, each plate 26,28 is characterized by a width (W) and wall 14 is characterized by aheight (H), wherein the width W of plates 26, 28 is greater than theheight of the wall. Of course, to the extent plates 26, 28 are diskshaped, then any references to width W should be replaced by a diameterD. However, for purposes of the low profile description, whether case 12is circular in shape and characterized by a diameter D or otherwisecharacterized by a height H, the low profile characteristic is the same.While not limiting the overall low profile, in one embodiment, theheight H is no more than 50% of the width W or diameter D. In onenon-limiting example, the height H of pod 10 is approximately 6.5 inchesand the width/diameter of pod 10 is approximately 18.5 inches.

As shown in the drawings, the pod 10 is substantially externallysymmetrical about its x and y axes, such that, when deployed, pod 10 cansettle on either side 30, 32 and still effectively couple to the oceanbottom. Thus, the orientation of pod 10 becomes much less of a concernas compared to prior art OBS systems designed to settle on the bottom inonly one “upright” position. Furthermore, because of the narrow profileof pod 10, its balance is generally unstable on edge 34. Thus, to theextent pod 10 touches down on the ocean bottom on edge 34, the pod 10will tip over and settle on one of the two faces 30, 32.

Pod 10 also includes internal ribbing 33 used to support plates 26, 28as pod 10 is subjected to the high pressures characteristic of an oceanenvironment. Ribbing 33 prevents any “rattle” or movement of plates 26,28 that could otherwise interfere with seismic wave detection. Unlikethe prior art, pod 10 as described herein is effectively a casing forthe geophones such that a seismic wave can pass undistorted through thepod's plate to geophone 18. In this regard, because of the low profileand rigid nature of pod 10, the attachment point of geophone 18 withincase 12 becomes of less consequence and the problems associated withprior art designs are overcome.

Each unit may include a unique identification means, such as a radiofrequency identification (RFID) tag 40 or similar identification indiciato permit tracking of the individual units as they are handled on thedeck in the manner described below. Likewise, each unit may include anacoustical location transducer 42 which permits the unit's location onthe ocean floor to be determined.

FIG. 1 also shows a hydrophone 44 to permit measurement of pressure anda connector 46 for permitting communication with pod 10 when pod 10 ison deck or otherwise disposed in a rack as described below. Connector 46may be a standard pin connector or may be an infrared or similarconnector that requires no hard wiring in order to communicate with pod10. Via connector 46, pod 10 may be serviced without removing one ofplates 26, 28 or otherwise opening case 12. Specifically, connector 46permits quality control tests to be run, recorded seismic data to beextracted, clock 20 to be synchronized and power source 22 to berecharged. Because connector 46 is only utilized above the water, awater tight, pressure resistant connector cap 47 may also be provided toprotect connector 46. Utilizing such a connector cap 47, connector 46may be any standard connector that satisfies the desired functions ofthe pod. Connector 46 need not be of the type normally required ofexternal connectors subjected to high pressure, corrosive environments.

Finally, shown in FIG. 1 is an optional attachment bracket 48 forclamping or otherwise grasping and manipulating pod 10. Bracket 48 ispositioned on case 12 so that the radial angle between bracket 48 andany hardware that may be extending from pod 10, such as transducer 42 orhydrophone 44 is obtuse or acute. In the embodiment shown, the angle isacute. Specifically, it is common that upon deployment or retrieval ofdevices such as pod 10, such devices may bang against the side of theship or other equipment as the pods are manipulated, potentiallydamaging hardware that protrudes from the devices. By positioningbracket 48 on the periphery of case 12 so that the radial axis extendingfrom the center of case 12 through bracket 48 is less than 90° separatedfrom the radial axis extending from the center of case 12 throughtransducer 42, the likelihood of damage to this hardware is diminished.

In one embodiment of the invention, rather than incorporating anattachment bracket 48, a latching mechanism is attached to wall 14,again, preferably, in an position to minimize damage to equipmentprotruding from pod 10. One effective latching mechanism is anover-center latching mechanism having opposing jaws that can be openedand closed to permit the units to be attached to a cable for deployment.The latching mechanism may further be attached askew to wall 14 so thatthe major axis of the latching mechanism and the z-axis of the pod 10 donot intersect. Again, such an orientation further protects hardwareprotruding from pod 10.

In FIG. 4, the external surface 50 of one or both of plates 26, 28 isillustrated. Specifically, surface 50 may be provided with projections51, such as ridges or grooves, to enhance coupling between pod 10 andthe ocean floor. In the embodiment shown, the projections 51 form achevron pattern on surface 50.

Also shown on FIGS. 4 and 5 is an attachment bracket 54 which may beincorporated for clamping or otherwise grasping and manipulating pod 10so that plates 26, 28 remain substantially horizontal as pod 10 islowered through a water column by a cable attached bracket 54. As such,bracket 54 may be axially centered on one of plates 26,28 or otherwisepositioned on one of plates 26, 28 above the center of gravity of pod10.

Turning to FIGS. 4-8, one of the aspects of the invention is theincorporation of a bumper, generally numbered as bumper 52, around thepod 10. FIGS. 4-8 illustrate three different configurations of bumper52, wherein the configurations are referred to as bumper 52 a, bumper 52b and bumper 52 c. In any event, bumper 52 has several functions. First,it may be shaped to urge pod 10 onto one of the two faces 30, 32 whenpod 10 touches down on the ocean bottom on edge 34. Bumper 52 alsofunctions to protect pod 10 and any external devices, such as transducer42, which may be protruding from case 12. Finally, the bumper may be ofa shape that inhibits pod 10 from becoming entangled by shrimping netsand shrimping drag or “tickle” chains. In any case, bumper 52 may servesome or all of these functions.

As stated above, bumper 52 may have several designs. In FIG. 5, bumper52 a is shown in cut-away disposed around case 12, while in FIG. 4, abumper 52 a is seen in a top view of pod 10. Specifically, bumper 52 ais shown as having a rounded or curved cross section 55. As shown,bumper 52 a includes a shoulder 56 which fits into a groove 58 definedaround the periphery of case 12. A portion 60 of bumper 52 a extendsbeyond the periphery of case 12, thereby protecting edge 34 of case 12.Due to the rounded nature of the bumper 52 a, pod 10 will roll or tiltonto a coupling surface of plates 26, 28 if pod 10 begins to settle onthe ocean floor so that plates 26, 28 are perpendicular with the oceanfloor. Furthermore, bumper 52 a will function to protect pod 10 fromshock and to protect personnel during handling of pod 10.

An alternate bumper profile is shown in FIGS. 6 and 7 in which bumper 52b has a wedge-shaped cross-section 62. Again, bumper 52 b includes ashoulder 56 which fits into a groove 58 defined around the periphery ofcase 12. A portion 64 of bumper 52 b extends beyond the periphery ofcase 12, thereby protecting plates 26, 28 and edge 34 of case 12. Thebumper 52 b illustrated in FIGS. 6 and 7 also includes cavities 66 whichcan be utilized as handholds for grasping and, manipulating pod 10. Inthe embodiment of 52 b, it can be appreciated that it is desirable toorient pod 10 having bumper 52 b on the ocean floor so that the wedge ofbumper 52 b faces down. Thus, for this embodiment, plate 28 isconsidered the top of pod 10 and plate 26 is considered the bottom ofpod 10.

In the bumper 52 b embodiment of FIGS. 6 and 7, an additional bumperportion 68 is shown mounted on top plate 28. Bumper portion 68 has arounded cross-section 70 that transitions into wedge-shapedcross-section 62. In one embodiment, glass beads may, be molded orotherwise incorporated into bumper portion 68 to increase the buoyancyof bumper portion 68. By increasing the buoyancy at the top of pod 10,this insures that pod 10 will be properly oriented, i.e., so that wedgeshaped bumper 52 b faces down, as pod 10 passes through a water columnand settles on the ocean floor.

To the extent a chain or other line is pulled against pod 10 when it iscoupled to the ocean floor, the chain will simply slide along thewedge-shaped surface of bumper 52 b and up over the top of pod 10.Bumper portion 68 further prevents such a chain or line from snagging orcatching on any equipment which may be protruding from the upward-facingplate surface of pod 10.

Still yet another embodiment of bumper 52 is illustrated in FIG. 8 inwhich bumper 52 c is comprised of a flipper or wedge 72 having a narrowend 74 and a wide end 76. Wide end 76 is fitted and hinged between twobrackets 78 attached to wall 14 of case 12. Preferably, brackets 78 areshaped so that their out edge 80 forms a substantially smooth transitionsurface with the surface of wedge 72. During deployment, pod 10 cansettle on either surface 26, 28 and the hinged wedge 72 will flap downagainst the ocean floor, forming a ramp or skirt over which a shrimperchain or similar line will ride when pulled against pod 10. In this waybumper 52 c will urge the chain over the top of pod 10 preventing thechain from snagging or catching pod 10.

FIG. 9 illustrates the flexible, non-rigid cable 82 of the invention.Specifically, cable 82 is comprised of an inner core 84 and an outercasing 86. Inner core 84 is formed of non-rigid material. For purposesof the application, non-rigid material means stranded or fibrous,non-conducting material such as rope. It has been found that syntheticfiber material is preferable although other materials can serve thepurpose of the invention. In one non-limiting example, the syntheticfiber is polyester. In one embodiment, core 84 is comprised ofindividual rope strands 88 formed of twisted rope fibers, wherein therope strands 88 are braided together to form core 84. Outer casing 86 ismolded over core 84. Casing 86 is further provided with ribs or grooves90 to reduce drag in the water. In one embodiment, outer casing 86 isformed of polyurethane.

It will be appreciated that since pod 10 requires no externalcommunications or power, cable 82 can be formed of a non-conductivematerial. Cable 82 as described herein is high strength with low stretchand no creep. Unlike rigid cable of the prior art, cable 82 does notexhibit torque, i.e., twisting, under load. Furthermore, cable 82 islight weight and easy to handle, especially compared to rigid andsemi-rigid cable of the prior art. Thus, utilizing cable 82, pods 10 canbe deployed along a receiver line by attaching pods 10 along cable 82 atspaced intervals.

As illustrated in FIGS. 9 and 10, one aspect of the invention is tosegment the cable and utilize a break-away connector 92 between cablesegments 94. Connector 92 is comprised of a first fitting 96 that isseated inside a second fitting 98. A shear pin 100 is inserted throughthe fittings 96, 98 to secure the fittings together. The fittings areattached to the adjacent free ends of cable 94 using any standard means.In one embodiment, each of fittings 96, 98 has a bore 102, 104,respectively, extending from the first end 106 to the second end 108. Atsecond end 108, each fitting has an aperture 97, 99 passing throughopposing sides of each fitting. When fitting 96 is seated inside secondfitting 98 such that apertures 97, 99 are aligned, shear pin 100 fitsthrough the aligned apertures 97, 99, joining fittings 96, 98 at therespective second ends 108.

Defined within each bore 102, 104 at their respective first ends 106 isa shoulder 110. Each fitting is inserted over the free end of a cable 98and a stop 112 is attached to the cable so that stop 112 abuts shoulder110 and holds the fitting on the end of the cable. In anotherembodiment, the bore extending from second end 108 to first end 106 maytaper and a stop larger than the diameter of the tapered bore can beutilized to secure the fitting on the free cable end.

In any event, each fitting 96, 98 is attached to the end of a cablesection 94 such that when the fittings are secured together, the cablesections form a longer length of cable. If the tension in the longerlength of cable becomes greater than the shear limit of the shear pin,the shear pin will break away and the longer length of cable willseparate. Because the shear pin is easily inserted and removed, theshear limit for the joined cables can easily be adjusted for aparticular environment or situation. For example, a shear pin with ashear limit of 5000 lbs may be desirable under certain conditions,whereas a shear pin with a shear limit of 8000 lbs may be desirable inother instances. To the extent the connector is separated under a shear,once the cable is retrieved, the fittings can easily be reattached byreplacing the broken shear pin.

Such a break-away system is desirable because a cable tensioned beyondits operating limits can snap. For example, in prior art rigid andsemi-rigid cables, tensions of 30,000 lbs or more can sometimes begenerated. A cable snapping under such a load is likely to result indamage and injury. It is much more desirable to simply retrieve a lengthof separated cable than to incur such damage and injury.

In another aspect of such a system, the break-away tension of the podsattached to the cable is higher than the break-away tension of theconnectors attaching cable segments. Thus in the event of a break awaytension, the cable, segments will separate before a pod is separatedfrom the cable. This is desirable because it is much easier to locateand retrieve a length of cable, which can be snagged, than it is tolocate and retrieve an individual pod which may have separated from thecable.

FIGS. 12 and 13 illustrate a clamping mechanism 120 that permits seismicunits to be clamped directly on a length of cable without the need tocut the cable as required in many prior art devices. Clamping mechanism120 includes a sleeve 122 with an axial bore 123 therethrough thatpermits sleeve 122 to be fitted over a cable (not shown). Clampingmechanism 120 also includes overmolded shoulders 124, 126 disposed onopposite ends of sleeve 122. An aperture 128 passes through each end ofsleeve 122, preferably in both the x and y planes perpendicular to theaxis of sleeve 122. In the illustrated embodiment, sleeve 122 includes aring portion 130 to which a seismic unit may be attached. In anotherembodiment, sleeve 122 may be tubular without a ring portion 130. Sleeve122 may be integrally formed or may be halves clamped together such asis shown in FIG. 13, where a sleeve first half 132 and a sleeve secondhalf 134 are clamped around a cable (not shown) and secured to oneanother with fasteners 136.

When installed on a cable, a pin is passed through apertures 128 tosecure clamping mechanism 120 from sliding on the cable. Shoulders 124,126 are molded over the ends of sleeve 122 and help secure theattachment pins in place. The ends of sleeve 122 may also be flared tohelp secure shoulders 124, 126 in place.

Thus, rather than cutting a cable and attaching a clamping mechanismbetween free cable ends, the sleeve of the invention can be clamped orslid onto a length of cable and secured in place without cutting thecable. Using pins to secure the mechanism in both the x and y planesprevents rotation of clamping mechanism 120 relative to the cable andprevent slippage axially along the cable.

The back deck of a seismic system deployment and retrieval vessel isillustrated in FIGS. 14-19. Generally shown in FIG. 14 is a seismicsystem deployment and retrieval vessel 200 having a work deck 202 with aseismic deployment and retrieval system 204 disposed thereon fordeploying and retrieving cable 206.

One component of the deployment and retrieval system 204 is a storagerack 208 for storing the OBS units attached to cable 206. As will beappreciated, storage rack 208 is scalable to meet the particular podstorage needs and space limitations of a vessel. In FIGS. 14 and 15,four storage racks 208 have been provided to maximize the pod storagecapacity of the particular vessel 200. As best seen in FIG. 16, eachstorage rack 208 is comprised of multiple rows 210 and columns 212 ofslots 214, wherein each slot 214 is disposed for receipt of a pod 216.While the dimensions for slot 214 may vary depending on the dimensionsof the particular OBS unit stored therein, the preferred embodimentillustrates storage rack 208 disposed for receipt of low profile, diskshaped pods as described above and generally referred to as pod 10.Referring to FIG. 17, each slot 214 is provided with a communicationsportal 218 to permit communication between a pod 216 and a mastercontrol station (not shown) when pod 216 is seated in slot 214. In oneembodiment, communications portal 218 is linked with pod 216 via theconnector 46 shown in pod 10 (see FIG. 1). As described above, the linkmay be a hard wire between communications portal 218 and connector 46 ormay be some other method of communication, such as an infraredconnector. Whatever the case, through portal 218, information recordedon the pod 216 can be downloaded, the unit batteries can be recharged,quality control checks on the unit can be conducted, the clock can besynchronized, recording can be re-initiated and the unit can bere-activated, all while seated in slot 214.

In another embodiment of storage rack 208, the rows and columns of slotsare replaced by a single stacked column of carousels, preferablysemicircular or u-shaped. Each carousel includes rollers to permit therecording units to be moved along the path of the carousel in conveyortype fashion until the units are positioned adjacent a communicationsportal. The shape of the carousel path is preferably semicircular oru-shaped to permit recording units to be inserted at a first end of thecarousel and removed from a second end. Such a configuration wouldpermit pods to be inserted and removed simultaneously from the carousel.As an example, the first end of the carousel may be located next to acleaning station for cleaning pods retrieved from the ocean floor andthe second end of the carousel may be located next to a deploymentstation to permit pods to be reattached to the cable for deployment.

Whichever storage system is utilized, the storage systems may beconfigured to have the dimensions of a standard 8×20′×8′ shippingcontainer so that the storage systems and any seismic units storedtherein, can be easily transported utilizing standard container ships.

As best seen in FIGS. 15, 17 and 18, one embodiment of system 204 isshown in which the back deck system is substantially automated.

In addition to the storage rack 208, there is shown a pod deploymentsystem 219 running adjacent the racks 208 and extending to the edge ofthe deck 202 adjacent the water. A pick and place system 220 ispositioned for moving the units 216 between the storage rack 208 and thedeployment system 219. While various automated and semi-automated pickand place systems 220 may be utilized, in the embodiment shown, one ormore single axis shuttles 221 are used to move pods 216 between one ormore grappling arms 223 that can move pods 216 between racks 208,shuttles 221 and the deployment system 219.

More specifically, deployment system 219 is comprised of a conveyorroller bed 226 running parallel to non-rigid cable 206 and a poddeployment carriage 228 moving in conjunction with conveyor 226. A cableengine 222 and cable spool/container 224 are positioned to linearly movenon-rigid cable 206 adjacent the deployment system 219 and over the sideof the vessel. Pods 216 are attached to non-rigid cable 206 while cable206 continues to be paid out into the water, i.e., on-the-fly, byutilizing carriage 228 to accelerate pod 216 to the speed of cable 206.At the point when the velocity of cable 206 and pod 216 aresubstantially equivalent, pod 216 is attached to cable 206, at whichpoint pod 216 is released from carriage 228 and continues to move alongconveyor 226 propelled by the cable to which it is attached.

Conveyor 226 has a first end 230 and a second end 232, wherein the pickand place system 220 is positioned adjacent the first end 230 and one ormore cable engines 222 are positioned adjacent the second end 232, suchthat pod 216 generally travel along conveyor 226 from the first end 230to the second end 232. Pod deployment carriage 228 likewise runs on atrack or frame 234 at least partially along a portion of the length ofconveyor 226. When a pod 216 is ready for deployment, it is pulled fromrack 208 utilizing arm 223 and moved on shuttle 221 to a positionadjacent the first end 230 of conveyor 226. A grappling arm 223 placespod 216 on carriage 228 which is likewise positioned on its track 234 tobe adjacent first end 230 of conveyor 226. Once pod 216 is in place oncarriage 228, carriage 228 is accelerated down conveyor 226 towards thesecond end 232 of conveyor 226. As the acceleration of the carriage 228reaches the velocity of cable 206, pod 216 is clamped or otherwisesecured to cable 206. In one embodiment, pod 216 includes a clamp withjaws that can be closed around cable 206 once attachment speed isattained. In such an embodiment, pod 216 can be clamped directly ontocable 206 or can be clamped to an attachment sleeve disposed on cable206. In either case, cable engine 222 will continue to pull cable 206,causing pod 216 to move down conveyor 226 until it is deployed over theedge of boat 200.

One or more RFID readers 240 may be placed along pick and place system220 and deployment system 219 to track movement of particular pods 216along deck 202. Such tracking is particularly desirable with respect tothe deployment and retrieval system 204 described above because theself-contained nature of the pods eliminates the need to keep units in aparticular order as they are manipulated on deck 202 and inserted intoracks 208. In other words, since the individual pods 10 of the inventionare self-contained and each pod's ocean floor location and orientationinformation is recorded within the pod along with the seismic datarecorded at the location, the units need not be kept in sequential orreceiver line order as they are retrieved from the ocean, manipulatedand stored. In this regard, units that might have been adjacent oneanother on the shot line need not be moved in a particular order throughsystem 204 and need not be stored next to one another in racks 208, butmay be randomly inserted into the storage rack 208.

As can be appreciated by those skilled in the art, the speed of thecable 206 as it is paid out into the water is constantly adjusted tocompensate for the erratic and unpredictable movement of vessel 220 inthe water. In the preferred embodiment, the speed of the carriage 228carrying the units 216 for attachment to the cable 206 can continuallybe adjusted to permit pod 216 to be smoothly attached to cable 206 onthe fly.

While conveyor 226, carriage 228 and cable 206 are all described in alinear arrangement, it is understood that non-linear arrangements arealso encompassed by the invention, so long as such arrangementsaccelerate a marine seismic unit so as to permit attachment of the unitto a moving cable.

As described above, deployment system 219 can be utilized to practiceone method of the invention, namely attachment and release of seismicunits 216 on the fly without stopping the movement of cable 206 as it ispaid out into the water. The method which can be used in conjunctionwith deployment system 219 includes the steps of providing a cablemoving at a given speed and along a cable path, accelerating a seismicunit along a path adjacent to the cable path until the seismic unit ismoving at approximately the speed of the cable and attaching the seismicunit to the cable while both are in motion. In this way, a seismic unitcan be attached to a cable and released into the water without the needto stop and start the cable and/or the vessel during deployment, therebyreducing the time necessary to lay out a length of cable along areceiver line.

In another embodiment of the invention shown in FIG. 19, asemi-automatic conveyor 250 intersects with the cable 206 as it is beingpulled from cable spool/container 224 and paid out by the cable engine222. In this case, storage racks 208 and pick and place system 220 arearranged on either side of conveyor 250, in a configuration similar tothat shown in FIG. 15. However, rather than having cable 206 runadjacent conveyor 250, cable 206 is spaced apart from conveyor 250. Inthis embodiment, conveyor 250 is defined by a first end 252 and a secondend 254. A portion 256 of conveyor 250 is curved to permit pods 216 tobe moved out to cable 206 for attachment of pods 216 to cable 206 at thesecond end 254 of conveyor 250. Also shown is a second conveyor 258 usedto stage pods 216 prior to attachment to cable 206. Second conveyor 258moves pods 216 from a position adjacent the pick and place 220 to thefirst end 254 of conveyor 250.

An attachment station 260 is defined at the intersection of cable 206and conveyor 250. At attachment station 260, a marine seismic unit 216is attached to the cable 206 and the attached unit is subsequentlyreleased into the water. In one embodiment, a cable grabber 262 ispositioned downstream from the attachment station 260. During deploymentof pods 216, cable grabber 262 is used to securely clamp cable 206 priorto attachment of a unit 216 at attachment station 260, thereby removingline tension upstream of grabber 260 to permit a unit 216 to be safelyattached to cable 206. This is especially desirable in semi-automatedconfigurations in which personnel manually attach units 216 to cable206. In any event, a cable grabber release system 264 may be included atattachment station 260 to minimize the likelihood that personnel areadjacent or in contact with cable 206 at the time cable grabber 262 isreleased and cable 206 is placed under tension. In the preferredembodiment, release system 264 includes a first button 266 and a secondbutton 268 that must be simultaneously actuated in order to cause arelease by cable grabber 262. Thus, desirably, a single operator mustuse both hands in order actuate release system 264 and as such, releasesystem 263 functions as a safety device to minimize danger to theoperator.

While not necessary, in the embodiment of the invention illustrated inFIG. 19, the back deck is outfitted with two cable deployment systemswherein one system is located on the port side of deck 202 and the othersystem is located on the starboard side of deck 202 with storage racks208, pick and place system 220 and conveyor 250 positioned therebetween.Conveyor 250 curves out to both sides and each cable deployment systemincludes a cable spool/container 224, a cable engine 222, an attachmentstation 260 and a cable grabber 262. Dual systems such as this permitredundancy and ensure that the seismic operation will not be delayed inthe event of malfunction of one of the systems.

One function of the seismic data recording unit of the invention is thecontinuous operation of the unit. In this aspect of the invention, dataacquisition is initiated prior to positioning of the unit on the earth'ssurface. In one preferred embodiment, a marine seismic unit is activatedand begins acquiring data prior to deployment in the water. Systems thatare activated and begin acquiring data prior to deployment are therebystabilized prior to the time when signal detection is desired. Thisminimizes the likelihood that an altered state in electronics operationwill disrupt signal detection. Of course, in the case of a continuousdata acquisition unit such as this, the novelty lies in the “continuous”nature of the unit and such function is applicable whether on land or ina marine environment.

In a similar embodiment, data recording is initiated prior topositioning along a receiver line. For example, a marine seismic datarecording unit is activated while still on the deployment vessel andbegins acquiring data prior to deployment in the water. Again, thispermits units to stabilize prior to the time signal recording isdesired. To this end, one component of system stabilization is clockstabilization. Of the various components of the system, it is well knownthat clocks typically take a long time to stabilize. Thus, in oneembodiment of the invention, whether the unit is continuously detectingdata or continuously recording data, the clock always remains on.

In either of the preceding two methods, the unit can be utilized inseveral cycles of deployment and retrieval without interrupting thecontinuous operation of the unit. Thus, for example, prior todeployment, recording is initiated. The device is deployed, retrievedand redeployed, all while recording is continued. As long as memory issufficient, this continuous recording during multiple cycles ofdeployment and redeployment can be maintained.

In this regard, to the extent the seismic data unit includes wrap aroundmemory, it can continuously record even when not in use in seismicdetection. Thus, in addition to the advantages described above,initiation or start instructions become unnecessary. Further, continuousrecording utilizing wrap around memory functions as a back-up for dataacquired from prior recordings until such time as the prior data iswritten over. An additional advantage is that the device is ready fordeployment at any time as long as the clock is synchronized.

To the extent recording is continued after a unit has been retrieved,routine operations such as data collection, quality control tests andbattery charging can take place without interrupting recording. Onebenefit of such a system is that the device can be utilized to recordquality control test data rather than seismic data when conductingquality control tests. In other words, the data input changes fromseismic data to quality control data. Once quality control is complete,the device may resume recording seismic data or other desired data, suchas data related to position and timing.

In one preferred embodiment of the invention, a marine seismic unitincludes an inertial navigation system to measure the unit's x, y and zposition information as the unit is passing through the water column andsettles on the ocean floor. Generally, such a system measures movementin each of the x, y and z dimensions as well as angular movement aroundeach x, y and z axis. In other words, the system measures the sixdegrees of freedom of the unit as it travels from the vessel to theocean floor, and utilizes such measurement information to determinelocation on the ocean floor. In the preferred embodiment, such x, y andz dimensional information can be determined utilizing accelerometers.Angular orientation, i.e., tilt and direction, information can bedetermined utilizing a tilt meter and a compass or other orientationdevices, such as gyroscopes. In one embodiment of the invention, threeaccelerometers and three gyroscopes are utilized to generate theinertial navigation data used to determine the unit's ocean floorposition.

In any event, by combining accelerometer and the tilt and directioninformation as a function of time with the unit's initial position andvelocity at the time it is discharged into the water column, the travelpath of the unit through the water column can be determined. Moreimportantly, the location of the unit at the bottom of the water column,i.e., the location of the unit on the ocean floor, can be determined.Time sampling will occur at appropriate intervals to yield the accuracyneeded. Time sampling between various measurement components may vary.

For example, data from the compass, used to measure direction, and thetilt meter, used to measure tilt, may be, sampled more slowly than datafrom the accelerometers. Heretofore, no other marine seismic unit hasutilized one or more accelerometers to determine location in this way.In this regard, the method and system replaces the need to determineocean floor location utilizing other techniques, such as throughacoustical location transducers or the like.

Notwithstanding the foregoing, this position determination methodfunctions particularly well with the above described continuousrecording method. Because a unit is already recording data as it isdischarged into the top of the water column, x, y and z positionalinformation is easily recorded on the unit and becomes part of theunit's complete data record.

The invention also provides for a unique retrieval method for OBS units300 attached to a cable 302, as illustrated in FIG. 20. Specifically, ithas been found that retrieving cable 302 over the trailing end 304(generally the stern) of a vessel 306 as the vessel moves leading end308 (often the vessel bow) first down a cable 302 in the direction ofthe cable minimizes dragging of the cable on the ocean floor 310 as thecable 302 is taken up and prevents undue tension or “pulling” of thecable 302 common in the prior art retrieval technique. Specifically, thewater drag on the OBS units and cable in the method of the inventioncauses the cable 302 to parachute or billow out behind vessel 306, asshown at 312, utilizing the water column as a shock absorber andminimizing undue tension.

In this method, regulation of the speed of the vessel 306 is not ascritical as in the prior art over-the-bow retrieval method. Furthermore,because the cable 302 is billowed out 312 in the water behind the vesselas the vessel moves in the opposite direction from the billow, the cableis less likely to become entangled in the vessel's propellers as mayoccur using the prior art method. Of course, those skilled in the artwill understand that in the method of the invention, cable can be takenup over the bow or the stern Of the vessel as long as the vessel ismoving in a direction along the cable and the cable is being taken up bythe trailing end of the vessel.

In any event, a flotation release system 314 may also be attached to thecable, generally at one or both ends of the deployed cable, to cause atleast a portion of the cable to rise to the surface where it can beeasily snagged for retrieval utilizing the above described method. Sucha system is well known in the art and may include a flotation devicethat is released from near the ocean floor at the desired time ofretrieval or a flotation device that floats on the water surface butremains attached to the cable while deployed.

The non-rigid cable of the invention is also incorporated in a uniquedeployment method for the pods, as illustrated in FIG. 21. Specifically,at least two OBS units 400 are tethered together using a non-rigid cable402. The cable 402 and units 400 are deployed into a water column 404.Because the units 400 are of a much greater negative buoyancy than thenon-rigid cable 402, the units will have a tendency to sink through thewater column ahead of the cable such that the cable segment adjoiningtwo units parachutes between the two units as shown at 406. The drag ofthe cable down through the water column functions as a break, slowingthe descent of the units and permitting the placement of the units onthe ocean floor 408 to be more readily controlled. Specifically, theparachuting effect permits control of the orientation of units such asthose outfitted with the wedge shaped bumper illustrated in FIGS. 6 and7, furthermore, the non-rigid cable cause the unit to gently settle onthe ocean floor, allowing for consistent coupling of the units to theocean floor.

This is an improvement over the prior art methods because the prior artmethods utilize a rigid or semi-rigid cable for deployment of OBS units.Such cable has a tendency to sink quickly through the water column alongwith the units. In other words, such cables do not have the same dragcharacteristics as the lighter weight, non-rigid cable of the invention.In cable and OBS units utilizing this prior art method, the orientationof individual units is much more likely to destabilize, e.g., wobble offcourse or flip over, as the unit quickly passes through the watercolumn.

An additional benefit to the deployment method of the invention is thatthe non-rigid cable permits slack to form between adjacent units, bothduring deployment and once settled on the ocean floor. In fact, it hasbeen found that during general deployment operations such as describedabove, the length of the non-rigid cable between two units willgenerally be much greater than the actual spacing between the units onceresting on the ocean floor. In other words, once settled on the oceanfloor, there may be a great deal of slack in non-rigid cable betweenadjacent units. For this reason, the non-rigid cable of the invention isnot utilized to space units apart from one another. In any event, avessel operator can utilize the slack that forms in the non-rigid cableto cause correction to a receiver line as it is being laid.Specifically, if a deployment vessel drifts or otherwise causes areceiver line being laid to be positioned off of the desired receiverline, the vessel at the surface can reposition to cause the remainder ofthe non-rigid cable and attached units to begin settling back on thedesired receiver line. The slack in the cable resulting from thenon-rigid nature of the cable permits the operator to get back on lineand cause the remainder of the individual units to settle inapproximately their desired location along the intended line. Incontrast, if such units were attached to a rigid or semi-rigid cable,the cable would not have any adjustment slack and the remainder of theunits, while perhaps positioned along the desired receiver line, wouldnot be positioned in the desired location along the receiver line.Furthermore, once the units 400 are in position on the ocean floor, thecable 402 between them is slack, as shown at 410. This “decouples”individual units from one another and prevents strumming or transmissionof undesired noise along the cable.

To the extent clock 20 is a crystal clock, information from the tiltmeter 38 may be used to correct for gravitational effects on clocktiming. In the prior art, tilt meter information has only been used tocorrect seismic data. Other than crystal clock corrections to accountfor temperature effects, no other type of crystal corrections have beenmade to such clocks. Thus, one aspect of the invention utilizes tiltmeter information to correct inaccuracies in the clock timing arisingfrom gravitational effects acting on the crystal clock. Such clockcorrection can be carried out on-board the pod at or near the time ofdata recording, or applied to the data once the data has been extractedfrom the pod.

Likewise, information from the tilt meter 38 can be used to applymathematical gimballing to the seismic data. To the extent seismic datahas been corrected in the prior art to adjust for orientation, suchcorrection has been based on mechanical gimbals installed on board theprior art OBS systems. However, a typical mechanical gimbal can causedeterioration in the data fidelity due to dampening of the gimbal in itscarriage. In one aspect of the invention, it has been determined that anon-gimballed, mathematical correction, or “mathematical gimballing” isdesirable over the gimballing methods of the prior art. Thus, theinvention may utilize tilt meter information to mathematically adjustthe seismic data to account for vertical orientation of the pod. Suchmathematical gimballing can be carried out on-board the pod at or nearthe time of data recording, or may be applied to data once it has beenextracted from the pod.

In addition, information from compass 36 can be used to further refinethe mathematical gimballing to account for rotational orientation of theunit. Specifically, compass data can be incorporated with the tilt meterdata in mathematical gimballing to more fully correct seismic data foreffects arising from orientation of a pod.

What is claimed is:
 1. An ocean bottom seismic data acquisition unit torecord seismic data while on an ocean bottom, comprising: at least onegeophone disposed within the ocean bottom seismic data acquisition unitto continuously monitor for seismic signals while on the ocean bottom,wherein the at least one geophone is not mechanically gimballed; atleast one clock disposed within the ocean bottom seismic dataacquisition unit; at least one power source disposed within the oceanbottom seismic data acquisition unit; at least one memory disposedwithin the ocean bottom seismic data acquisition unit to store theseismic data while on the ocean bottom; at least one accelerometerdisposed within the ocean bottom seismic data acquisition unit; theocean bottom seismic data acquisition unit having negative buoyancy andthe ocean bottom seismic data acquisition unit configured for retrievalfrom the ocean bottom; and wherein the seismic data acquisition unitdoes not receive or transmit an external communication while on theocean bottom.
 2. The ocean bottom seismic data acquisition unit of claim1, wherein the ocean bottom seismic data acquisition unit includes aunique identifier.
 3. The ocean bottom seismic data acquisition unit ofclaim 1, wherein the ocean bottom seismic data acquisition unit includesa radio frequency identification tag.
 4. The ocean bottom seismic dataacquisition unit of claim 1, comprising: a water tight case, the watertight case housing the at least one geophone, the at least one clock,the at least one power source, and the at least one memory.
 5. The oceanbottom seismic data acquisition unit of claim 1, comprising: a watertight case, the water tight case defining at least one internal watertight compartment.
 6. The ocean bottom seismic data acquisition unit ofclaim 1, wherein the at least one power source is rechargeable,comprising: a connector configured to allow electronic access to the atleast one power source to recharge the at least one power source on adeck of a vessel.
 7. The ocean bottom seismic data acquisition unit ofclaim 1, wherein the ocean bottom seismic data acquisition unit includesexactly three geophones.
 8. The ocean bottom seismic data acquisitionunit of claim 1, wherein a width of the ocean bottom seismic dataacquisition unit is greater than a height of the ocean bottom seismicdata acquisition unit.
 9. The ocean bottom seismic data acquisition unitof claim 1, comprising: at least one tilt meter.
 10. The ocean bottomseismic data acquisition unit of claim 1, wherein the at least one clockis an atomic clock.
 11. The ocean bottom seismic data acquisition unitof claim 1, wherein the ocean bottom seismic data acquisition unitcomprises a disk-shaped case.
 12. A system to record seismic data whileon an ocean bottom, comprising: a first ocean bottom seismic dataacquisition unit comprising: at least one geophone disposed within thefirst ocean bottom seismic data acquisition unit to continuously monitorfor seismic signals while on the ocean bottom, wherein the at least onegeophone is not mechanically gimballed; at least one clock disposedwithin the first ocean bottom seismic data acquisition unit; at leastone power source disposed within the first ocean bottom seismic dataacquisition unit; at least one memory disposed within the first oceanbottom seismic data acquisition unit to store the seismic data while onthe ocean bottom; at least one accelerometer disposed within the firstocean bottom seismic data acquisition unit; the first ocean bottomseismic data acquisition unit having negative buoyancy and the firstocean bottom seismic data acquisition unit configured for retrieval fromthe ocean bottom; wherein the seismic data acquisition unit does notreceive or transmit an external communication while on the ocean bottom;and a cable attached to the first ocean bottom seismic data acquisitionunit disposed on the ocean bottom and attached to a second ocean bottomseismic data acquisition unit disposed on the ocean bottom.
 13. Thesystem of claim 12, wherein there is slack in the cable between thefirst ocean bottom seismic data acquisition unit disposed on the oceanbottom and the second ocean bottom seismic data acquisition unitdisposed on the ocean bottom.
 14. The system of claim 12, wherein thefirst ocean bottom seismic data acquisition unit comprises a disk-shapedcase.
 15. A method of acquiring seismic data using an ocean bottomseismic data acquisition unit while on an ocean bottom, comprising:continuously monitoring for seismic signals via at least one geophonedisposed within the ocean bottom seismic data acquisition unit on theocean bottom, wherein the at least one geophone is not mechanicallygimballed; recording the seismic data on at least one memory disposedwithin the ocean bottom seismic data acquisition unit on the oceanbottom; wherein the seismic data acquisition unit does not receive ortransmit an external communication while on the ocean bottom; whereinthe ocean bottom seismic data acquisition unit has negative buoyancy andthe ocean bottom seismic data acquisition unit is configured forretrieval from the ocean bottom; and wherein the ocean bottom seismicdata acquisition unit further comprises: at least one clock disposedwithin the ocean bottom seismic data acquisition unit; at least oneaccelerometer disposed within the ocean bottom seismic data acquisitionunit; and at least one power source disposed within the ocean bottomseismic data acquisition unit.
 16. The method of claim 15, comprising:coupling a surface of the ocean bottom seismic data acquisition unitwith the ocean bottom.
 17. The method of claim 15, wherein the oceanbottom seismic data acquisition unit continuously monitors for seismicsignals without receiving or transmitting external communications. 18.The method of claim 15, wherein the ocean bottom seismic dataacquisition unit records the seismic data without receiving ortransmitting external communications.
 19. The method of claim 15,comprising: utilizing the at least one accelerometer to acquireaccelerometer information on the ocean bottom.
 20. The method of claim19, comprising: storing, on the at least one memory, the accelerometerinformation on the ocean bottom.
 21. The method of claim 15, wherein theocean bottom seismic data acquisition unit includes a sensor, the methodcomprising: determining a location of the ocean bottom seismic dataacquisition unit on the ocean bottom based on information obtained fromthe sensor.
 22. The method of claim 21, wherein the ocean bottom seismicdata acquisition unit remains attached to a cable while on the oceanbottom.
 23. The method of claim 15, wherein the ocean bottom seismicdata acquisition unit comprises a disk-shaped case.